The minimum stellar metallicity observable in the Galaxy

TL;DR

This study investigates the lowest metallicities in Galactic halo stars to infer the initial mass function of the first stars, finding evidence for predominantly very massive Population III stars with masses below approximately 140 solar masses.

Contribution

It combines observational data and theoretical models to constrain the primordial IMF by analyzing metal-poor stars and assessing the effects of interstellar accretion on their surface abundances.

Findings

Surface abundances are minimally affected by accretion.

Most first stars were very massive but likely below ~140 solar masses.

Metal-poor stars reflect early Universe chemical conditions.

Abstract

The first stars fundamentally transformed the early Universe through their production of energetic radiation and the first heavy chemical elements. The impact on cosmic evolution sensitively depends on their initial mass function (IMF), which can be empirically constrained through detailed studies of ancient, metal-poor halo stars in our Galaxy. We compare the lowest magnesium and iron abundances measured in Galactic halo stars with theoretical predictions for the minimum stellar enrichment provided by Population III stars under the assumption of a top-heavy IMF. To demonstrate that abundances measured in metal-poor stars reflect the chemical conditions at their formation, and that they can thus be used to derive constraints on the primordial IMF, we carry out a detailed kinematic analysis of a large sample of metal-poor stars drawn from the SDSS survey. We assess whether interstellar accretion has altered their surface abundances. We find that accretion is generally negligible, even at the extremely low levels where the primordial IMF can be tested. We conclude that the majority of the first stars were very massive, but had likely masses below ~140 M.